Two common (non-type-specific) antigens in HCl extracts prepared from group A streptococcal cultures of different types were analyzed by immunodiffusion methods. These antigens are sensitive to trypsin and are apparently related to the cell wall proteins. The common antigens tested belong to a category of nonprotective non-type-specific antigens. The presence of common antigens in the HCI extracts should be taken into consideration when M-proteins are determined. The detection of one of the common antigens may be used as a virulence index, since this antigen is characteristic of the group A streptococcal cultures with enhanced virulence.

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